This invention was made with United States Government support under a contract with the United States Navy. The federal government has certain rights in this invention.
Generator rotors have a large diameter cylindrical body from which extends at both ends a smaller diameter shaft. The rotor body has a series of longitudinal slots cut deep into its outer circumference. In these slots are inserted field windings that extend the length of the rotor body. There are rotor wedges in the slots that hold the windings in place against centrifugal forces exerted when the rotor rotates. These rotor wedges are above the windings in the rotor slot.
The end turn portions of the windings extend axially out beyond each end of the rotor body. These end turns electrically connect the longitudinal section of a winding in one slot with a similar winding section in another slot. As the rotor spins, the end turns are thrust radially outward by centrifugal force. This radial movement of the end turns is confined by cylindrical retaining rings that constrain the end turns.
The retaining rings slide over the ends of the, rotor body. They are usually attached to the ends the rotor body by shrink fitting. The rim of the retaining ring is shrink fitted tightly around a circumferential lip on the end of the rotor body. In addition, locking keys securely hold the retaining rings onto the rotor body to prevent axial movement of the rings. These keys fit in opposing grooves the retaining ring and in both the rotor teeth and wedges. Without these keys, thermal expansion of the field coils and the retaining rings may cause the retaining rings to slide axially off the rotor.
In some applications, high frequency currents exist on the surface of the rotor body and retaining rings. For example, when the generator is used in conjunction with a load commutated inverter (LCI), cycloconverter (CCV) or other non-linear load, eddy currents are induced on the surface of the rotor. These rotor eddy currents are the results of harmonics of the input and/or output currents of the LCI and CCV devices. These eddy currents have high frequencies and, thus, primarily reside at and near the surface of the rotor and retaining rings.
Losses due to eddy currents in the rotor result in undesirable I.sup.2 R (Joulean) heating. Accordingly, low resistance harmonic current paths are needed to reduce eddy current losses and thus minimize heating. The rotor body and retaining rings, typically made of high strength steel alloys, are not themselves good electrical conductors. The relatively high electrical resistance of the retaining rings and rotor body to the eddy currents will cause high losses and heating. To avoid these losses, eddy current shields cover the outer surfaces of the rotor body and retaining rings to provide a low resistance electrical path for surface eddy currents. The shields reduce losses of eddy currents and prevent localized heating of the rotor body and retaining rings.
An eddy current shield is commonly a thin copper layer applied to the outer surfaces of the rotor body and retaining rings. The shield can be a jacket or a cladding. In the alternative, an eddy current shield for the rotor can be provided by specially configured wedges made of conductive chromium-copper or other alloys that have wings that overhang the rotor teeth adjacent the wedge.
While eddy current shields substantially reduce current losses, they do not reduce the magnitude of the eddy current. The current shields merely provide low resistance paths for the currents. Any interruption or point of high resistance in these current paths will cause additional loss and localized heating. The gap between the outer surface of the rotor body and the inner surface of the retaining ring is an interruption to eddy current and a long time aggravating source of eddy current losses. Accordingly, it is desirable to provide a reliable low resistance electrical connection between the rotor body and retaining rings.
In addition, galling occurs in prior art devices between the top surface of the wedges in the rotor slots and the retaining rings. Generally, galling occurs on the mating surfaces of the wedges, rotor slots and retaining rings. When frictional movement occurs between these mating parts, localized hot spots and even spot welding can arise. Especially at rotor standstill, prior art wedges can be subjected to extremely high pressure (for example, on the order of 5000 psi) if the shrink fitted retaining ring unintentionally pushes the wedge against the keyed surfaces of the rotor slot.
Because of the wedge crushing pressure that can result when a retaining ring shrinks onto a wedge, wedges have been typically designed so as to be at or below the surface of the rotor where the retaining ring covers the rotor. The compression of the wedge down into the slot is most severe when the rotor is stationary and centrifugal forces do not counteract the forces from the overlapping retaining ring. Since the wedges do not fit perfectly within their key slots in the rotor, the wedges can move slightly in the slot. Small gaps exist between the wedges and retaining ring. These gaps close as the wedges are forced radially outward by the centrifugal forces of rotor rotation. The opening and closing of the gaps between the wedge and retaining ring creates undesirable wedge chattering and causes galling between the wedges and both the rotor slots and retaining rings.
The current invention solves the wedge crushing pressure problem of retaining rings and provides a reliable electrical contact for eddy currents between the surface of the wedges and the retaining ring. The key slots in the rotor allow a top step on the wedge surface to extend radially beyond the rotor teeth surface where the retaining ring overlaps the wedge and rotor. The key slot also allows the wedges to move radially in the slot. The rotor wedges are biased radially outwardly by springs, e.g., arc leaf springs, immediately underneath the wedges. The springs flex under the pressure from the retaining ring and relieve the pressure from the retaining ring on the wedges, especially at rotor standstill when there are no counteracting centrifugal forces. In addition, these springs bias the wedges against the inner surface of the rim of the retaining ring to provide an electrical path for harmonic eddy currents.
It is an objective of this invention to reduce galling between rotor slots, wedges and retaining rings. Another object is to provide a reliable electrical junction between a rotor and a retaining ring for high-frequency eddy currents from standstill to operating speed. In addition, it is an object of this invention to provide such a junction by means of a spring mounted underneath rotor wedges to bias the wedges against the inner surface of the overlapping end of the retaining ring.